How to diagnose seronegative Antiphospholipid Syndrome (APS)?

Diagnosing Seronegative Antiphospholipid Syndrome (APS)

Seronegative APS should be considered in patients with clinical manifestations of APS but negative conventional antiphospholipid antibody testing, and diagnosis requires testing for non-criteria antibodies such as anti-phosphatidylserine/prothrombin (aPS/PT) antibodies. 1

Clinical Approach to Suspected Seronegative APS

Step 1: Establish Clinical Criteria Suggestive of APS

• Vascular thrombosis: Objectively confirmed arterial, venous, or small vessel thrombosis in any tissue or organ
• Pregnancy morbidity:
  • One or more unexplained fetal deaths (≥10 weeks gestation)
  • One or more premature births (<34 weeks) due to severe preeclampsia/placental insufficiency
  • Three or more consecutive spontaneous abortions (<10 weeks)

Step 2: Confirm Negative Conventional aPL Testing

• Ensure thorough testing for all three conventional aPL has been performed:
  • Lupus Anticoagulant (LA)
  • Anticardiolipin antibodies (aCL) - IgG and IgM
  • Anti-β2-glycoprotein I antibodies (aβ2GPI) - IgG and IgM
• Testing should be repeated at least 12 weeks apart to confirm persistent negativity 2
• Ensure testing was performed before anticoagulation was started, as anticoagulants can interfere with LA testing 1

Step 3: Consider Non-Criteria aPL Testing

For patients with strong clinical suspicion of APS but negative conventional testing, consider testing for:

1. Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies:

   • Most promising non-criteria marker for seronegative APS
   • Available as ELISA tests for both IgG and IgM isotypes
   • Prevalence in seronegative APS with thrombotic manifestations: 9% for IgG and 5% for IgM 1
   • Particularly useful when LA testing is unreliable due to anticoagulation 1

2. Anti-Domain I β2GPI antibodies (aDI):

   • Can be used as a confirmatory test for specificity of aβ2GPI antibodies
   • May help with risk stratification in suspected APS 1

Step 4: Evaluate for Alternative Causes

• Rule out other causes of thrombophilia (Factor V Leiden, Prothrombin gene mutation, etc.)
• Consider other autoimmune conditions that may mimic APS

Interpretation of Non-Criteria aPL Results

• Positive aPS/PT: May suggest a false negative LA result, particularly in patients with double aPL positivity (aCL and aβ2GPI) 1
• Negative aPS/PT: May suggest lower thrombotic risk 1
• aPS/PT and LA association: May confer increased risk for cerebrovascular events, even when β2GPI-dependent tests are negative 1

Important Considerations and Pitfalls

• Timing of testing: Antibody levels may fluctuate during pregnancy or acute thrombotic events. Repeat testing post-delivery or at a distance from acute events 1
• Anticoagulation interference: LA testing is prone to interference from anticoagulants; aPS/PT measured by solid phase assays may be useful when LA testing is unreliable 1
• Standardization challenges: Detection of aPS/PT has proven difficult to standardize, and reference material is lacking 1
• Risk of overdiagnosis: Including aPS/PT in first-line diagnostic workup may lead to potential overdiagnosis of APS, as these antibodies can be found in other autoimmune diseases 1

Management Implications

If seronegative APS is diagnosed based on clinical criteria and non-criteria antibodies:

• Long-term anticoagulation with vitamin K antagonists (target INR 2.0-3.0 for venous events)
• Management of traditional cardiovascular risk factors
• For pregnant women: low-dose aspirin plus prophylactic low molecular weight heparin until 6 weeks postpartum 2

The diagnosis of seronegative APS remains challenging and requires careful clinical judgment, as the prevalence of non-criteria antibodies in truly seronegative patients is relatively low. Future research is needed to better establish the role of these antibodies in APS diagnosis and risk assessment.